RELATIONSHIPS BETWEEN CARBON AND HYDROGEN ISOTOPE RATIOS AND NITROGENLEVELS IN LEAVES OF CLUSIA SPECIES AND 2 OTHER CLUSIACEAE GENERA AT VARIOUS SITES AND DIFFERENT ALTITUDES IN VENEZUELA

Samples of the Clusiaceae genera Clusia, Oedematopus and Dystovomita w ere collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hy drogen and of leaf-nitrogen levels were performed on the dried samples . Correlations among these variables, i.e. carbon isotope discriminati on (Delta), hydrogen isotope ratio (delta D) and N-levels, and with al titude were assessed. In the samples, where values of Delta above 15 p arts per thousand indicate predominant performance of C-3 photosynthes is, there were slight tendencies of increasing Delta, delta D and N-le vels with increasing altitude and of increasing Delta with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpir ation and increased leaf-nutrient supply at increasing altitude. Perfo rmance of crassulacean acid metabolism (CAM) in species of Clusia appe ars to be restricted to altitudes below 1500 m a.s.l. There was a sign ificant negative correlation of Delta with altitude in the samples, wh ere values of Delta below 10 parts per thousand indicated predominant performance of CAM. This suggests that phases II and IV of CAM are pro gressively suppressed towards the upper altitudinal limit of CAM in Cl usia in northern Venezuela. It is concluded that among the large numbe r of environmental factors and combinations thereof, which determine t he expression of CAM in Clusia and trigger C-3-CAM transitions in C-3/ CAM intermediate species, low availability of water is the most import ant.